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Hindawi Publishing Corporation International Journal of Volume 2013, Article ID 678621, 9 pages http://dx.doi.org/10.1155/2013/678621

Review Article The Development of as a Tool in Marine Biological Research in the Twentieth Century

John A. Fornshell1 and Alessandra Tesei2

1 National Museum of Natural History, Department of Invertebrate Zoology, Smithsonian Institution, Washington, DC, USA 2 AGUAtech, Via delle Pianazze 74, 19136 La Spezia, Italy

Correspondence should be addressed to John A. Fornshell; [email protected]

Received 3 June 2013; Revised 16 September 2013; Accepted 25 September 2013

Academic Editor: Emilio Fernandez´

Copyright © 2013 J. A. Fornshell and A. Tesei. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The development of acoustic methods for measuring depths and ranges in the environment began in the second decade of the twentieth century. The two world wars and the “Cold War” produced three eras of rapid technological development in the field of acoustic oceanography. By the mid-1920s, researchers had identified echoes from fish, Gadus morhua, in the traces from their echo sounders. The first tank experiments establishing the basics for detection of fish were performed in 1928. Through the 1930s, the use of SONAR as a means of locating schools of fish was developed. The end of World War II was quickly followed by the advent of using SONAR to track and hunt whales in the Southern Ocean and the marketing of commercial fish finding for use by commercial fisherman. The “” composed of invertebrates and fish was discovered in the late 1940s onthe echo sounder records. SONARs employing high frequencies, broadband, split beam, and multiple frequencies were developed as methods for the detection, quantification and identification of fish and invertebrates. The study of fish behavior has seen someuse of passive acoustic techniques. Advancements in computer technology have been important throughout the last four decades of the twentieth century.

1. Introduction research. Following the war, active acoustic ranging devices in the form of echo sounders began to be employed in During the twentieth century, the use of acoustics to study measuring ocean depths. Soon, acousticians began to rec- life in the was developed into a significant tool for ognize the ability to detect marine organisms, principally research in . The purpose of this paper is to fish, using these devices. The use of sound to detect fishas briefly recount the process by which the use of acoustics as a a tool in the fishing industry and fisheries research began biological research tool took place. The general pattern was a slow development from the mid-1920s until the out-break the development of acoustic technology for nonbiological of the Second World War. This conflict produced an even research uses, navigation and military operations to name larger stimulus for the development of knowledge about the two and then the application of that technology to the behavior of sound in the marine environment. Following detection and study of . By the end of the twentieth the Second World War, the development of SONAR as a century, acoustic technology had become a significant factor tool in biological research accelerated constantly. The “Cold in marine biological research. Marine biologists were devel- War” also led to the development of advanced knowledge of oping acoustic equipment for the specific purpose of studying ocean acoustics and sophisticated passive SONAR technology life in the oceans. which became available following the collapse of the Soviet The development of modern acoustic technologies for use Union in 1991. In this brief history, the developments are in the ocean environment began during the second decade chronicled in terms of the first uses of acoustics for biological of the twentieth century. The First World War provided a work prior to World War II; subsequent to this conflict, for significant stimulus for the advancement of ocean acoustics theperiodfrom1946to2000,marinemammalresearch, 2 International Journal of Oceanography biology research, and fisheries biology are each soundpropagationinthesea.Theacronym,SONAR(SOund treated as a separate thread in the development of SONAR Navigation And Ranging), was proposed by F. V. (Ted) as a tool in biological research. HuntofHarvardUniversityin1942andisthetermmost commonly applied to acoustic detection devices, both active 2. Early Developments and passive [1, 15]. This new found knowledge was quickly applied to more peaceful enterprises, the commercial whaling Many historians site the sinking of the RMS Titanic as and fishing industries in particular. the immediate stimulus for the development of underwa- ter acoustic ranging technology. In response to this event, 3. SONAR Applications in the Study of developed the “” to Marine Mammals detect icebergs and the floor. This device was first used on the US Coast Guard Cutter Miami in 1914 [1, 2]. The Following the end of World War II, 1946, surplus military threat of German during the First World War led SONAR units were fitted onto some whale catcher vessels to significant advances in underwater acoustic technology. of European nations and were used to hunt whales in the The most important contributions in the development of Southern Ocean. The use of this equipment greatly improved active acoustic detection devices were made by the team the efficiency of the whaler’s efforts in killing whales. The first of the French physicist, Paul Langevin, and the Russian commercial acoustic detection equipment for the hunting Constantin Chilowsky. These workers produced an acoustic of whales was being manufactured and marketed by the transmitter which could be mounted on a ship. Their initial Kelvin-Hughes Company in 1948. The use of SONAR to instrument emitted sounds at a frequency of 38 kHz (see actively track a whale with the technology available in the [1] for a complete summary of the principles of underwater late 1940s was limited to ranges of about 1 mile, 1600 m, with acoustics). By the year 1919, a device working in the frequency an optimal range from 450 m to 900 m. By 1956, more than range from 20 kHz to 50 kHz was being installed on warships. forty catcher boats from Norway, Great Britain, and Denmark R. W. Boyle coined the acronym ASDIC (Anti- were equipped with the Kelvin-Hughes Echowhale Finder. Detection Investigation Committee) for use in reference The installation of SONAR equipment on pelagic Japanese to underwater acoustic detection devices [1–3]. In 1922, whale catcher vessels began in 1958 and was universal by 1962. Harvey Hayes working on the USS Stewart, a navy destroyer, The installation of acoustic detection equipment in Japanese introduced the use of the echo sounder to measure water coastal whale catcher vessels began earlier, in 1950, but was depths; he ran a transect across the Atlantic Ocean from never in use on all members of the coastal whaling fleet16 [ ]. Newport, Rhode Island to Gibraltar, making a total of 900 In the case of baleen whales, the SONAR pings frightened depth measurements. Between 1923 and 1928, US Coast and the whales resulting in an escape behavior in which the Geodetic Survey vessels were fitted with deepwater sounding animals swam at high speed near the surface in a straight instruments. Similar equipment was being fitted on survey line away from the sound source. This caused them to tire shipsoftheRoyalNavyHydrographicOfficeandtheGerman more quickly and made it easier to follow the whale and Navy’s research ship Meteor. This marks the beginning of the kill it. In hunting sperm whales, which tend to dive to great use of acoustics in an organized fashion to study the marine depths when being pursued, the SONAR was used to track environment [4]. the animal during its dive and to position the catcher vessels Portier reported on the possibility of detecting sardines neartheplacewherethewhalesurfaces.Spermwhaleshave using an echo sounder [5]. In 1927, Rallier du Baty was been observed to modify their click frequency to match that thefirsttoattributefalseechoesonechogramrecordings of a tracking SONAR [16]. from the Grand Banks in the western North Atlantic as In 1963, Walker, using passive listening equipment, being indications of the presence of cod, Gadus morhua [6]. reported on the detection of underwater sounds with Two years later, in 1929, Kimura published the results of widespread geographic distribution, apparently of biological experiments in which he demonstrated that fish could be origin. The low-frequency, 20 Hz, sounds were detected in the detected with the relatively simple acoustic equipment of his North Atlantic and North Pacific Oceans. The trains of pulses time [7]. In 1935, Edgell using an echo sounder on the Royal with a period of approximately ten seconds were observed to Navy Hydrographic Office survey vessel HMS Challenger was (a) be point sources, (b) to move with a speed of five knots able to detect fish in the North Atlantic8 [ ]. In the mid-1930s, orless,and(c)tobesomewhatintermittent,thatis,tobe the use of echo sounders by fishing vessels began on a very punctuated by periods of silence. The conclusion that Walker limited scale [9]. Norwegian fisheries biologists used echo reached was that they approximated the sounds of a large sounders to survey cod populations in Vestfjord and, by 1937, mammal’s heartbeat. They would have been less audible while included acoustic surveys of herring [10–12]. Balls, who was the whale was on the surface and large marine mammals have captain of the herring drifter Violet and Rose, used the echo depressed heart rates when diving [17]. sounder on his vessel to improve the efficiency of his fishing During the “Cold War,” 1948 to 1990, the reliance on pas- effort [13, 14]. sive acoustic methods to detect and track submarines gained The use and development of acoustic methods for fish- favor and resulted in the expenditure of $15 billions over eries science were interrupted by the Second World War. The 40 years by the American defense establishment to develop period from 1939 to 1945, World War II, saw an extremely and deploy the SOund SUrveillance System (SOSUS). During rapid development of our understanding of the physics of World War II, it had been discovered that, at the depth in International Journal of Oceanography 3 theoceanwheresoundvelocityisminimum,soundenergy humpback whales, in the Northwest Atlantic Ocean between becomes trapped and travels for great distance. This “sound Iceland and the United Kingdom. The IUSS/SOSUS trans- channel”orSOFARlevelwouldallowalistenertohear ducer arrays are bottom-mounted allowing both direction sounds from great distances, tens of thousands of kilometers. and distance to the source to be determined. Spectrographic In the 1950s, the US Navy began building the SUSOS in the analysis of the recorded signals allowed the determination of “sound channel”. The system consisted of a number of large the species of the source animal(s) [25]. arrays mounted on the sea floor at the depth of Marine biologists were learning a great deal about the minimumsoundvelocity,theSOFARlevel.Bylinkingthese acoustic behavior of the whales using the partially decom- arrays into a network of listening stations, Soviet submarines missioned SUSOS. Humpback whale song components could couldbelocatedandtrackedeveniftheywerethousandsof be recognized reliably, although only the frequencies below miles away [18]. a few hundred Hertz were typically received. These were the During the 1980s, Surface Sys- songs associated with male reproductive displays, best known tem (SURTASS) was developed to complement the fixed from calving and breeding aggregations near shore. Although hydrophone arrays. This was a long string, up to 5000 m, blue, fin, and humpback whales have variable but generally of acoustic transducers towed behind an ocean surveillance similar source levels and frequencies in these calls, blue whale ship, thus allowing American and NATO navies to track callsoftenappearedtobereceivedoverlongerdistances.This Soviet submarines from the time they entered the North Sea suggests that these calls were more consistently produced without actively emitting sounds, which would have told the at depths that allowed better transmission via deep sound submariners that they were being tracked. The basic research, channels to the bottom mounted arrays. If the which facilitated this purely operational military project, fin and humpback whales were mating display calls, then they would ultimately lead to a number of research tools for would have been near the surface and in shallow water [20– biological, physical, and geological oceanographic research 25]. [18]. Duringthe“ColdWar,”1948to1990,therewasasteady In 1991, the navy largely decommissioned the SOSUS but decrease in the operating frequencies and an increase in left a small number of monitoring posts, three in the North the amplitude/energy being radiated by operational active Pacific Ocean, operational and made these available to the SONARs culminating in the AN/SQS 53A-C (A—army, N— scientific community. Different marine mammals produce navy, S—Sonar, Q—surface ship, and S—search), which vocalizations, which are characteristic of each species, allow- operated from 2.6 to 3.5 kHz with a power output of 235 dB ing them to be tracked and identified at significant distances. referenced to 1 𝜇 Pa at 1 m. These antisubmarine optimized An individual blue whale was tracked for 43 days using the SONARs had little biological applicability, although they SOSUS network [19]. The decommissioned SOSUS listening may have significant impact upon marine mammals due posts allowed cetacean biologist to track migrating whales in to the high power levels [2, 26]. In the mid-1990s, NATO the North Pacific Ocean, blue whales, Balaenoptera musculus, Centre for Maritime Research and Experimentation (CMRE, and fin whales, Balaenoptera physalus, observed using the called SACLANTCEN at that time and NURC more recently) seafloor array in the Northeast Pacific20 [ , 21]. started a research programme devoted to understanding The blue whale call sequences that were identified were the relationship between sound in the ocean and marine long series of repetitive down swept tonal calls with funda- mammal behaviour. A series of whale strandings in the mental frequencies usually below 20 Hz and several harmon- 1990s and early 2000s highlighted the need to understand ics, repeated variably from 3- to 10-minute intervals, often this relationship. Although navies throughout the world use over several hours. Blue whale sequences often were received sonar during tests and training exercises, little was known over distances of 500 km or more [22]. about the relationship between active sonar and marine Moore led a team of researchers using the largely decom- mammal strandings. The concern for environmental safety missioned SOSUS facility in the North Pacific and established led researchers at NURC to establish the SOLMAR (Sound, the source of the 20 Hz sounds first observed by Walker in Ocean and Living Marine Resources) project, later renamed 1963 as the calls of fin whales23 [ ]. Marine Mammals Risk Mitigation Project (MMRM), to The fin whale call sequences that were identified were investigate the phenomenon and attempt to find methods the repetitive down swept 20 Hz pulse series with the most of mitigating harm to whales in the sea when the loud energy at and a little above 20 Hz and little harmonic energy. low-frequency active SONARs are in use. This work, which Pulses were repeated regularly at rates of a few seconds in combined the use of tagging studies and visual sightings from characteristic temporal patterns with three to four rests of aircraftandshipswiththeuseofpassiveacousticmethods afewminuteseachhouroverperiodsof16hoursormore. for detecting whales and making observations over very wide These were the songs associated with male reproductive frequency range, led to new knowledge about whale [27– displays, best known from calving and breeding aggregations 30]. The vocalizations of Cuvier’s beaked whale, for instance, near shore [24]. were found to have the most energy between 20 and 60 kHz Similar work was carried out in the Northeastern Atlantic [31, 32]. The research conducted as part of the MMRM Project Ocean on humpback whales, Megaptera novaeangliae,when involved developing new technologies which among others Charif et al. used the Integrated Underwater Sound Sys- included towed arrays of passive transducers to detect whales. tem/SOund SUrveillance System (IUSS/SOSUS) to study the These towed passive acoustic arrays worked on the same seasonality of acoustic behavior and migration behavior of principles as the SURTASS described above. Such equipment 4 International Journal of Oceanography allowed acousticians to assess the risk to marine mammals in preservation changes the relative values of celerity and astudyarea.Italsoledtotheobservationthatsomewhales, density by very small amounts. These changes, however, are sperm whales, Physeter macrocephalus, may be present in the acoustically significant causing up to a 40% variation in the same waters as Cuvier’s beaked whales, Ziphius cavirostris, target strength (TS). He also noted that the TS varied from and not be affected by the high intensity sounds [27, 28, 31]. the predicted values for 𝑘𝑎 (𝑘 is the wave number, 2𝜋/𝜆, Acousticshavealsobeenusedinanefforttointerpret and 𝑎 is the characteristic cross section length) greater than some peculiar behavior of marine animals; a valuable exam- 1. It also varied significantly as the aspect of the animal ple is represented by the work conducted by Leighton et al. changed. Elongate species such as krill and Sergested shrimp in order to understand the reason why groups of humpback are directional reflectors, producing much larger TS when whales sometimes generate “bubble nets” in the form of a viewed dorsally or laterally as compared to frontal aspect. cylindrical curtain of shallow bubble cylinders to trap their This work was followed five years later with a paper published prey, which they then consume by rising from beneath. It by Greenlaw and Johnson presenting the values of the ratios is possible that the whales emit a sound which remains of sound velocity in the organism to that in and trappedwithinthebubblenet,therebyscaringthepreyand the ratio of the density of the animals to that of seawater preventing it from escaping [33]. for a wide variety of species using values for fresh/living specimens in seawater. This information was of significant use for researchers studying the acoustic signa- 4. SONAR Applications for the Study of tures of invertebrate animals [42]. This work was necessary Invertebrate Animals if marine biologists were to be able to identify acoustically detected planktonic animals. In the year 1946, depth recorders were showing the existence During the 1980s, acoustical research on micronekton of a “deep scattering layer” which migrated vertically on followed two different pathways. (1) Holliday and Pieper adiurnalcycle[34]. This phenomenon proved to be myc- developed multiple-frequency inversion methods to estimate tophid fish and planktonic organisms including physonect the acoustical size distribution from volume backscattering siphonophores, euphausids, and copepods which migrated data collected with several different frequencies of sound. vertically as a result of diurnal variations in solar illumination It has been met with significant success in estimating the [35–39]. Earlier workers had recognized acoustic signatures zooplankton and micronekton concentration in as having a biological origin, such as cod [6]. In the case of different acoustical size classes [40]. (2) Dual beam acoustical the deep scattering layer, the acoustic signature was observed methods, first developed in fisheries research in the 1970s, as an ocean-wide phenomenon and subsequently identified have been refined to allow the resolution and analysis of as biological in nature. Most of the observations were made at echoes from individual animals such as members of the small 12 kHz and the organisms proved to be invertebrate members macrozooplankton. Using this technology, the capability of of the zooplankton community. This marked the beginning of estimating the acoustical distribution of a zooplankton and the use of acoustics to discover and study marine biological micronekton assemblage was developed. When the results of topics that were new to science. An important step forward such an analysis are combined with the results of an echo was conducted by Stockhausen and Figoli in their 1973 study integration analysis of the corresponding volume backscat- using an efficient experimental method (and the appropriate tering data, estimates of numerical density and biomass sonar system, which was very advanced for that time) for concentration could be apportioned into different acoustical measuring volume scattering strength of the deep scattering size classes [43]. layers, illuminated from beneath. The methodology used Volume reverberation in the ocean is due in large part to explosive sources and a directional receiver placed in a biological causes at frequencies greater than 1 kHz. Working bistatic configuration in very deep waters, with the beam at Woods Hole Oceanographic Institution (WHOI), Wiebe, looking upward, so that the scattering layers were profiled building on earlier work by Stanton, found that backscatter- from below. This allowed a much higher depth resolution ing cross section increases with the volume of the animal. versus frequency, especially when, during the night, the His work also demonstrated that live specimens are stronger scattering layers lie very shallow [36]. acoustic targets than preserved specimens. Better approxi- The availability of higher frequency SONARs for use by mations were obtained with cylinder models as opposed to marine biologists in the 1970s led to the use of acoustics spherical models. The actual backscattering strengths are a for the study of invertebrates. Holliday and Pieper working complex function, which can be roughly approximated by on the problem of detecting and identifying smaller inverte- cylinder or bent cylinder models [44, 45]. brate animals developed the Multiple-frequency Acoustical Wiebe et al. showed that the key to using high frequency Profiling System (MAPS) which can be used to profile the sound in the study of zooplankton and micronekton was using the multiple-frequency methods they to deploy the acoustical transducer in a manner that gets pioneered in the 1970s and 1980s [40]. The device can also the transducer sufficiently close to the animals of interest. be towed behind a moving ship. This marked the beginning Thisisduetothefactthatattenuation(measuredindB/m) of acoustic research dedicated to studying invertebrates in the approximately increases with the square of the frequency. In pelagic environment. the case of the dual beam method, the range is further limited ∘ In 1977, Greenlaw measured the backscattering spectra by the need to resolve individual targets. Beam angles of 3 ∘ of preserved planktonic crustaceans [41]. The process of and 10 andafrequencyof420kHzwereused[45]. International Journal of Oceanography 5

The target strength of an organism can be plotted asa transducers in a circular pattern, or a split beam configuration function of frequency. In this case, the value of the parameter with the transducers divided into quadrants. The dual beam 𝑘𝑎 proves to be very important; below a value of 1, the slope configuration allows the observer to determine the angle of of target strength in dB/𝑘𝑎 rises very steeply with a slope of the targets, fish, to the axis of the beam. The split beam about 80; above a 𝑘𝑎 valueof5,theaverageslopeisnearlyhor- configuration allows the observer to determine the angular izontal. For most living organisms, this curve is not smooth position of the target, fish, to the observer. If the echo sounder but rather characterized by undulations due to constructive has a single beam, the target strength and range may be and destructive interference resulting from the external and displayed. If the echo sounder has a dual beam configuration, internal morphology of the target species. These undulations the target strength and relative angle to the axis of the beam may be used to identify the organisms being observed. aredisplayed.Asplitbeamconfigurationallowsthedisplayof This phenomenon was used by several workers to develop target strength and relative position to the echo sounder [53]. techniques for the acoustic identification of different taxa in As the years progressed, there was a steady shift from the sea. Multiple transducers, each operating at a specific adapting military and navigational hardware to the needs of frequency are one solution to this problem. Alternately, fisheries biology to the development of acoustic hardware appropriate wide band transducers may emit a continuous and methodologies specifically for the research needs of this wideband signal with a chirp modulated over a range of field. These research SONARs included multibeam SONAR, frequencies greater than ten percent of the center frequency. side scan SONAR, parametric SONAR, synthetic aperture The former method was used by Coombs and Barr in 2004 SONAR, and conventional low-frequency SONAR [53–56]. and the latter method was used by Chu and Stanton in 1998 These different types of SONARs may be combined to and by Stanton et al. [46–48]. facilitate specific research interests. The combination of side The underwater acoustic sensor BIo-Optical Multi- scan SONAR and echo sounder gives rise to the multibeam frequency Acoustic and Physical Environmental Recorder SONAR (generally exploiting the Mills Cross arrangement (BIOMAPER), developed at WHOI in the mid-1990s, repre- of a projector and a line hydrophone array) for high- sented a significant advance in the use of acoustics to study resolution, three-dimensional sensing of fish populations. [49]. In its initial form, transducers Using advanced computer data analysis methods, it is pos- operating at 120 kHz and 420 kHz allowed the detection of sible to construct a three-dimensional image of fish schools. organisms with characteristic acoustic cross sections on the This process allows the fisheries biologist to determine the order of centimeters. This meant that the acoustic device was length, width, vertical dimension, and position in the water detecting herbivores, euphausids mysid shrimp, Penaeidae column of a school of fish that has been detected. In some shrimp, primary carnivores, and small fish. Efforts to improve instances, this may allow the density estimation and the the system led to the use of seven different frequencies, identification of the species of fish in the schools when 38 kHz, 43 kHz, 120 kHz, 200 kHz, 420 kHz, 1 MHz, and sufficient knowledge of schooling behavior is available. In 2 MHz in BIOMAPER II. This refinement along with the addition, the avoidance behavior patterns of fish may be ability to vary the depth of the tow fish depth greatly quantitatively studied [57, 58]. enhancedthecapabilityofacousticallysamplingawider Active acoustics/SONAR using sound energy generated range of trophic levels including at least some of the primary by transducers to study fish populations dates from 1930s, as producers, planktonic algae [50, 51]. described earlier in this paper. The commercial application of such information was and still is a major factor in the 5. SONAR Applications for the Study of Fish development of active SONAR for fisheries research. The commercially available echo sounders available in the post- In the period between 1948 and 2000, the use of acoustics Second World War period were adequate for locating schools in fisheries biology increased rapidly. At this time, 1948, of fish and individual fish but were only marginally adequate the Japanese firm, Fururno Industries, began to market fish for basic research. Acoustic devices used for research may finders to the commercial fin fish industry. In Europe, the be monostatic, when a single transducer is used to both Kongsberg Simrad Corporation also marketed an increas- transmit and receive sound energy or when the transmitter ingly sophisticated line of SONARs with vertically steerable and receiver are colocated, or bistatic when the transmitter beams, multiple beams, and multiple frequencies, specifically and receiver(s) are separated by a distance large enough tailored to the needs of fishermen seeking fin fish in the North tobecomparabletothedistancetothetarget.Theformer Atlantic Ocean and adjacent from the early 1950s. In the configuration is generally less complicated to carry out last quarter of the twentieth century, these companies were in terms of both cost and geometrical/mechanical issues joined by Teledyne RESON A/S in the production of scientific and is used more often. When studying , the SONARs. transducer is mounted on the hull pointing downward, and Fishermen also make great use of the echo sounder to whenstudyingpelagicfish,itismountedsoastotransmit locate fish. In this case, the vertically oriented beam shows its beam horizontally. In both cases, the transducers consist thefishermanthedepthandbottomtopographyandmay of single or multiple piezoelectric elements which convert indicatethepresenceoffish.Thestrengthoftheechofrom electrical energy into acoustic energy and/or vice versa. If one the fish may be used to infer both the quantity of fish and, limits the description to directive sources, which are the most in some instances, the species [52]. The echo sounder may be commonly used in this field, the emitted “ping” spreads in single beam configuration, dual beam configuration with the a conical fashion similar to that of a flashlight beam, where 6 International Journal of Oceanography objects near the axis of the beam appear brighter than those In the 1970s, the accuracy of acoustic observations was off the axis. Similarly, objects near the axis of the acoustic greatly enhanced by the introduction of readily available beam are found to have greater acoustic amplitude than those inexpensive digital electronics. “Computers and modern off the axis. The edge of the main radiation beam (or lobe) is electronics have greatly increased our ability to collect, ana- defined as the distance between the axes and where the echo lyze, and display acoustical data. The fundamental measure- intensity is reduced to one-half, and twice this distances is the ment remains a time series of voltage and the grand challenge beam width. Also, the receiving beam is defined in the same is interpreting and translating these signals into information way [59–61]. useful for scientists and managers [60].” Measurements were Two basic measurements are derived from acoustic data: made more accurate because digital devices did not tend to backscatter from an individual target of interest, namely, drift as much as analog measurement equipment. Also, the target strength, and reverberation, which can be defined as introduction of small shipboard computers greatly speeded the persistent sound scattered and reflected after emission data processing [69]. The dependence of abundance estimates by all the elements of target’s environment [62]. When on backscatter is significant. An error of only three dB in organisms are dispersed, it is possible to obtain echoes from target strength will result in a factor of two differences in individuals, depending on the spatial resolution of the sonar abundance estimates [61]. The reflected sound from the swim compared to the animals’ size. Trout et al. first used this bladder has been found to account for 90% of the total energy method to obtain the numeric density, number of organisms backscattered by a fish depending on the frequency being per cubic meter. Most of the time, fish will form schools, used [70–74]. making echo counting impossible [63]. Medwin and Clay Developing a viable means of using acoustic data to 2 measured the acoustic backscattering cross section (bs/m ) identify different taxa was recognized as one of the great chal- and target strength (TS =20log10 relative acoustic pressure) lenges to the use of SONAR in fisheries biology and marine [62]. In this case, echo integration may be used in which the biology research [48]. Members of the zooplankton do not 2 volume backscattering/m is multiplied by the total volume have a and can be differentiated from fish which being sampled [64].Thissummationwasshowntobelinearly do have a swim bladder by using multiple frequencies, 12 kHz, proportional to the population density by Foote in 1983 [65]. 18 kHz, and 38 kHz are particularly useful for identifying The application of advanced signal processing methods macroscopic fish, whereas 120 kHz and 200 kHz are useful for and the enhanced understanding of the principles of sound zooplankters such as krill and shrimp [75, 76]. propagationintheseahaveincreasedthequalityandquantity Passive SONAR using acoustic energy produced by the of information obtained using acoustic methods. Interesting fish themselves has also been developed as a significant results of high-resolution population density imaging were research tool [59]. takes advantage of obtained through ultrasonic echo sounders; another valuable sounds produced by fishes to eavesdrop on their behavior. result was obtained by applying several independent acoustic Most fish sounds are associated with aggression, courtship, systems aimed to measure the low-frequency target strength and spawning [61].TheUSNavysponsoredalargescalestudy andpopulationdensityoftheAtlanticherring:thelow- of the fish sounds after World War II resulting in the catalog- frequency falloff in the response of schools of hundreds of ing of the sounds made by many different species of fish millions of animals was confirmed by the response of an [77]. Their work showed that a great number of fishes pro- individual and interpreted as being caused by the subres- duced species-specific sounds. The types of SONAR used onance scattering of the fish swim bladder [66, 67]. The in research in fisheries research include single hydrophones, largest sources of error in fisheries acoustic surveys are paired hydrophones, three, four, and five hydrophone arrays, incomplete coverage of the entire population, inaccuracy in sonobuoys, and towed arrays. The quantitative analysis of proportioning backscatter to taxa, and error in estimating sounds produced by fish and other marine animals is typically target strengths of individual organism [54]. by plotting signals in the time domain as oscillograms (ampli- Fisheries biologists using acoustics as a method of deter- tude versus time) and in the frequency domain (frequency mining the size of fish stocks developed empirical mod- versus time) as power spectra. Qualitative analysis is focused els (empirical models are statistical relationships between on characterizing biological sounds using terms such as observed and measured variables). In , chirps, whistles, and growls to name a few. By analyzing the most common relationship, target strength to length arrival times, phase shifts, from multiple hydrophones, the regression, is used to convert echo amplitude to fish lengths. direction to the organisms can be determined from interfer- Other common relationships are volume backscatter to abun- ometry. dance or biomass regressions [62]. The abundance of cod, While active SONAR has been widely adopted in the for example, could be determined by visually inspecting the post-Second World War era for estimating fish abundance, sum of signals as a function of the distance steamed. The passive acoustics has not seen widespread adoption. There are next step was automation of the process [68]. In parallel with three reasons for this: (1) the technology, both hardware and these developments, work by Cushing showed that the target software, has not been commercially available; (2) the general strength was greatly influenced by the presence and the size lack of ground-truthing data necessary to make quantitative of the swim bladder [60]. This work was important in that it assessments based on the acoustic data; (3) the sounds that began the process of differentiating between different species fish produce are often masked by the ambient noise in thesea basedonthetargetstrength[66]. [1]. International Journal of Oceanography 7

6. Conclusions References

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